Tonight I have two mortise gauges to look at. I bought one at Ed Lebetkin’s tool store above Roy Underhill’s Woodwright’s School. Chris Schwarz wrote about Ed’s stuff here: http://lostartpress.wordpress.com/2011/07/22/sometimes-temptation-comes-from-above/
I managed to get out of there with only this gauge. Just a few weeks ago, Jennie Alexander & I were talking about this model of gauge, and how it strikes a pair of lines that are staggered where they begin & end. It makes sense, and if we were to find a pair of lines like that on old furniture, we could say the joiner likely used a gauge like this one. (or a marking gauge struck twice) Well, in principal anyway.
The wear on this one, either on the beams that the pins are in, or on the pins themselves result in one pin striking a stronger line than its neighbor. Thus you need to go over the lines a bit, and that gets you a varied pattern of struck lines. they aren’t always consistent. So much for theory. But, it’s a nice mortise gauge. The beams have a small tongue & groove to keep them together, and the whole thing tightens with a wedge thru the head. The plan is to one day make one based on this…but that was the plan when I bought this next mortise gauge umpteen years ago.
This little one is more complicated than the previous one. It has two beams that nest together, and are captured in the head by a wedge. So far, so good. But one of the pins is installed in a block glued onto one beam. Then this block fits in a square notch cut in the other beam. Nice.
As an added registration, there’s a groove in one beam, and a small iron pin in the other to keep them keyed to each other.
Someone marked positions on the edges of the beams to line up the pins for specific mortises. Here’s the main setting – 3/8″
Shift things this way & you have 5/16”
and this way gets you 7/16”
all with two marks scribed across the beams. nice one.
But remember, these are more complicated than they need to be – here’s Alexander’s fixed gauge. one setting, over & over. https://pfollansbee.wordpress.com/2011/07/19/a-simple-mortise-gauge-and-some-mail/
PETER FOLLANSBEE: JOINER’S NOTES 
Those are both fascinating gauges. Are they both shop-made?
I’ve seen gauges like the top one before, and while I admire the simplicity of the design, I wonder about the practicality of the staggered pins when scribing a small mortise. It seems like you’d have to consistently over-scribe the lines, which would risk leaving layout lines on each side of the tenon when the joint is assembled.
I’ve never seen anything like the second one (but then, I’ve not been at this very long), but it’s ingenious. Personally, I kind of like the over-complicated movement from a purely aesthetic point of view. I’d be curious about the gauge’s practicality in everyday use. Apparently the initial owner got a lot of mileage out of it, at any rate.
Thanks for sharing.
Thanks for the comment. You have inadvertently given me a good laugh – “The risk of leaving layout lines” is my favorite new expression! As you will see by studying my work on the blog here, to me that’s no bother at all. Period workmanship didn’t worry about marks like that.
the smaller, more complicated gauge has been in everyday use in my shop for 10 years. lots of miles on it. works great. It’s carefully made, and I do still hope to make a copy. I keep thinking, “this winter…”
Glad I could amuse you, if nothing else. But it makes perfect sense that it would be appropriate for period work.
I’ve made a few marking gauges for myself, and have wanted to make an adjustable-pin mortise gauge, but I had never come across a design I really liked. I just might try to make a version of that second gauge. At some point…
Yup – it’s all in good fun. Let me know how you get on with that 2nd gauge, we’ll put it up here. It’s a beauty.
I really like the second gauge. (As you might guess, I’m an engineer.) I’d be tempted to run the wedge across the beam rather than with it, to minimize accidental movement when setting the wedge tight.
Mr Follansbee:-
Thank you very much for sharing these photos with us.
Well, I’ve finished my version of the second gauge:
I put my own touches on it, but all in all, I’m very pleased with the design.
[…] for shop-made mortise gauge, although I had never found one I liked until I ran across this one on Peter Follansbee’s blog: See the series of knife marks on the side of the arms? Line them up and you get precise, […]